Analyze a subject line
POST
/api/v3/letters/subject-line-analyses
const url = 'https://api.tryletterhead.com/api/v3/letters/subject-line-analyses';const options = { method: 'POST', headers: {Authorization: 'Bearer <token>', 'Content-Type': 'application/json'}, body: '{"api":true,"subject":"Your newsletter subject line here","isTopPerformer":true}'};
try { const response = await fetch(url, options); const data = await response.json(); console.log(data);} catch (error) { console.error(error);}curl --request POST \ --url https://api.tryletterhead.com/api/v3/letters/subject-line-analyses \ --header 'Authorization: Bearer <token>' \ --header 'Content-Type: application/json' \ --data '{ "api": true, "subject": "Your newsletter subject line here", "isTopPerformer": true }'Send a subject line to Letterhead’s AI for a short, plain-language critique of how well it’s likely to perform — clarity, personalization, urgency, curiosity, length, and tone.
This is a single-subject-line analysis, generated fresh on each call. For historical performance
data across your actual sends, see listSubjectLinePerformance instead — that endpoint returns
real open-rate figures with no AI analysis.
Authorizations
Section titled “Authorizations ”Request Body required
Section titled “Request Body required ” Media type application/json
object
api
Required. Set to true to authenticate with an API key.
boolean
subject
string
isTopPerformer
boolean
Example
{ "api": true, "subject": "Your newsletter subject line here", "isTopPerformer": true}Responses
Section titled “ Responses ”Subject line analysis
Media type application/json
object
items
object
analysis
string
message
string
total
integer
Example
{ "items": { "analysis": "The subject line \"Your newsletter subject line here\" likely performed well due to its clear personalization, making recipients feel directly addressed and engaged. While it doesn't employ urgency or curiosity, its straightforwardness and relevance suggest that subscribers recognized it as directly related to their interests, enhancing open rates. The medium length keeps it concise, while the informative tone assures recipients of its value, contributing positively to their decision to open the email." }, "message": "Do something kind today", "total": 1}Still can’t find what you need? Contact support.